The present invention relates to an optical measurement probe for process monitoring, in particular for reflection measurements on solids, emulsions and suspensions.
In the field of process apparatuses, optical measurement probes are frequently used to monitor processes in the process industry, in particular in the chemical, pharmaceutical and food industries. These measurement probes can be used to measure the concentration, material identity, turbidity and purity of starting materials, intermediate products and products (solids, emulsions and suspensions) in real time.
Optical measurement probes have the advantage that they operate without taking samples, enable the simultaneous determination of the concentrations of a plurality of analytes and can also be employed in unfavourable milieus (toxic, corrosive, radioactive, at risk of explosion, sterile, contaminated).
These probes are generally fibre-optic elements, whose distal ends, which have the light entrance opening, are arranged in the process apparatus, i.e. more or less near the analytes or in direct contact therewith, and whose proximal ends are coupled to an evaluation device, for example an NIR spectrometer.
These probes are generally used to carry out reflection measurements of the process material with the aid of a light source with a known spectrum, whose light is frequently coupled into the measurement site via a separate optical waveguide arranged in the measurement probe.
It is thus possible, for example, during the process control of polymer melts in the extrusion to quickly and reliably determine the exact chemical composition of a polymer melt in real time.
Time-consuming off-line analysis by taking samples can therefore be dispensed with.
Quite an accurate picture of the process conditions can thus be gathered by combining these measurement variables obtained by optical means with further measurement variables (temperature, pressure, pO2 etc.), not obtained by optical means, and it is possible to correctively intervene in the process in real time. Operational losses on account of production downtimes or malfunctions can be avoided in this manner.
Said systems comprise one or more fibre-optic measurement probes and an evaluation device, such as an NIR spectrometer, for example, and are available for example from Bayer under the trade name “Spectrobay”. Another supplier is Sentronic.
Said measurement probes must be of very robust and resistant design at least in the region of their distal ends because of the extreme chemical, thermal and mechanical conditions prevailing in the said process apparatuses. They therefore generally have a fibre-optic core and flexible metallic reinforcement. In order to bring about the required stability, generic measurement probes currently on the market have a diameter of at least 8 mm, which extends up into the distal region of the measurement probe.
The main problem with such measurement probes is, however, their high sensitivity with respect to contaminations. Material from the process apparatus, for example, has a tendency to deposit on the light entrance opening at the distal end of the measurement probe and can thus corrupt the reflection measurements. Such corrupted measurements must be expected in particular if the light entrance opening is situated in a reduced-flow region of the process apparatus, that means if deposits, once they have formed, are not easily swept away again, and/or if the material in the process apparatus has thermoplastic properties and, once it has deposited on the light entrance opening, solidifies on account of cooling.